It’s the time of year when bright curtains of colored light –auroras --dance across the northern sky, especially between 60 and 72 degrees north and south latitude. Aurorae, named for the Roman goddess of dawn, are a natural phenomenon occurring when charged particles, mostly electrons from the sun, enter the earth’s atmosphere where they are directed toward the polar regions by the Earth’s magnetic field. As these particles enter the upper atmosphere, they collide with other atoms and molecules. During the collision, some energy is transferred to the atom or molecule which becomes excited and re-emits the energy as light as it returns to a non-excited state.

The beautiful light displays are called aurora borealis (Boreas is the Greek god of the north wind) in the northern hemisphere and aurora australis (the Latin word for from the south) in the southern hemisphere. Although aurorae happen year-round, they are usually more visible in winter because of long periods of darkness.

In simple terms, aurora intensity and frequency depends on the solar wind, the stream of charged particles emanating from the sun’s upper atmosphere. More solar wind means more charged particles that enter the earth’s atmosphere which means more aurorae. Sunspots, which are temporary, appear as dark areas on the sun’s surface and amplify the solar wind. The result is more auroral activity on earth. Solar wind is stronger with more sunspot activity, which peaks about every 11 years.

Aurora borealis are best viewed on the northern horizon on a clear night. Colors are stratified and depend on altitude, composition and density of the atmosphere because different gasses emit different colors of light when charged with energy (a principal also used with neon lights). Aurorae are typically greenish-white due to the characteristics of oxygen in earth’s atmosphere. At higher altitudes, oxygen changes and emits a red light more easily captured on film than seen with the naked eye. The bottom edge, about 100 km altitude, may be purple from a mix of red and blue emissions from nitrogen molecules.

Scientists can predict aurora activity, but not with very high accuracy. In fact, predictions are not even as good as weather forecasts. Solar wind takes a couple of days to reach earth’s atmosphere so scientists make predictions based on the sun’s behavior, but this is difficult to do in a global sense let alone for one location.

Get your daily (and hourly) aurora forecast from the University of Alaska’s Geophysical Institute http://www.gi.alaska.edu/AuroraForecast and to see what’s up in space check spaceweather.com.—Marcy Davis